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Phase control technology of optical DQPSK receiver

Abstract

For 40Gbit/s and higher optical communication system, the differential quadrature phase shift keying (DQPSK) becomes the preferential modulation method recently due to its high-spectrum efficiency and higher dispersive tolerance, but this modulation method should control the phase error of the receiver to be within 1°. Compared to the differential binary phase shift keying (DBPSK), its control accuracy is increased by 5 times. We propose a new phase control method without dither to successfully realize the above target. This method overcomes the main implement obstacle of DQPSK technology and applies the DQPSK modulation format from the lab into the actual products.

Solution

When the phase of the DQPSK receiver is not at the optimum value (+/- 450), the signals received in this branch (SA in the synoptic diagram) not only include the data information (dA) in this branch, but also include data information in the side branch (dB), namely cross-talk of dB to dA, so the system performance reduces. The amount of this cross-talk is directly associated with the phase error. If we can extract dB from SA, we can monitor the phase error of the receiver and control the phase at the optimum value. Considering that dA is independent of dB in the branch A and B, the cross-correlation of SA and dB (multiplier and average in the synoptic diagram), mA indicates the amount of dB in SA. The analysis indicates that mA is proportional to the phase deviation of the branch A. It not only provides the amount of the phase deviation, but also provides the direction. Consequently, mA can be used as the feedback signals for phase control without dither.

For details, please refer to:
33rd European Conference and Exhibition on Optical Communication, September 16-20, 2007, Berlin, Germany. (ECOC2007) Paper: 3.5.2

Technical keys

  1. Monitor the phase deviation of the receiver by using the cross-correlation between the signals prior to branch decision and signals after side branch decision to control the phase without dither.
  2. Add low-pass filter and reduce signal bandwidth prior to the cross-correlation to reduce hardware implementation difficulty and improve robustness of the control method.

Synoptic diagram

dqpsk

Effect

  • Realize 0.20 phase control accuracy under stable operation.
  • Can tolerate 10,000ps/nm chromatic dispersion and still work without recovered clock.
  • Used in the world first commercial DQPSK optical communication system by Fujitsu.

Contact Information:

Tao Zhenning: taozn@cn.fujitsu.com

Li Lei: lilei@cn.fujitsu.com

Wu Jianming: jianming.wu@cn.fujitsu.com